Profile extrusion molding is a popular method for producing continuous uniform thermoplastic items having often complex cross sections for use as, for example, automobile exterior trim. The profile assumes the shape of the extrusion die of choice, and is cut and end-capped to form such articles as body side moldings. A single profile may be designed to fit many models of cars, making profile extrusion a popular after market process. A wide processing range of thermoplastic materials permits high outputs in profile extrusion. Continuous operation of an extruder permits uniform production of plastic products. The temperature required along the extruder barrel, adapter and die depend upon the specific extrusion process being conducted and the nature of the plastic used.
In a typical extrusion process, resin pellets are either gravity fed or force fed from a hopper into single or twin screw extruders and are conveyed along the screw surface. Solid and liquid additives, such as reinforcing additives and fillers, are often incorporated into the mix, and must be homogenously combined via distributive or dispersive mixing by the screw. Additives, however, can increase the viscosity of the polymer melt, thereby impairing the rate of extrusion. Additives that are difficult to disperse may be precompounded with molten polymer into pelletized concentrates. However, many such additives do not achieve even distribution throughout the melt, creating profile products having defects, such as uneven color or unreinforced points susceptible to stress.
Glass fibers are commonly used as reinforcing agents in conventional extrusion processes. Cut glass fibers may be added to the polymer melt, often at levels as high as 30% by weight. As the melt flows along the screw flight, these anisotropic glass fibers tend to align themselves parallel to the extrusion barrel in the direction of flow. Consequently, the resultant profile is unidirectionally strong in the longitudinal aspect. The plane transverse to the longitudinal direction is relatively weak, and thus relatively more prone to fracture upon impact. In addition, if the added glass fibers are not evenly dispersed during mixing of the melt, the degree of longitudinal strengthening of the profile will vary. Uneven, unidirectional reinforcement of this nature is highly undesirable in profile extrusion products used.
To overcome the disbursement problems associated with extrusion utilizing glass fiber, the process of pultrusion was developed. Pultrusion is characterized by pulling continuous glass strands through a tube containing a polymer matrix, thereby forming a reinforced continuous length rod having a constant cross section. A significant drawback to pultrusion is the unexpected presence of voids or channeling within the reinforced rods, due to entrapment of air bubbles during the pulling process. Extruded articles having such voids have low impact resistance and are susceptible to fracture.
Therefore, a need exists for improving the impact resistance of extrusion molded articles for use as, for example, automobile exterior trim.
An object of the invention is to provide a profile extrusion molded product having nearly uniform reinforcement in all aspects. The profile is formed from a material comprising at least one thermoplastic, and about 2% to about 15%, by volume, of reinforcing particles. The particles each comprise one or more layers, wherein at least 50% of the reinforcing particles are less than about 20 layers thick, at least 99% of the reinforcing particles are less than about 30 layers thick, and the layers comprise platelets having a thickness of between about 0.7 nm and 1.2 nm. Profiles comprising essentially homogeneously dispersed reinforcing particles in the given amounts have a uniformly reinforced cross section.
It is a further object of the invention to provide a method of manufacturing a highly reinforced extrusion molded part that is equally reinforced in all aspects as discussed above. In accordance with this object, the present invention provides a method of producing a reinforced article comprised of a continuous extrusion profile having good strength in all directions. The method prepares a melt of at least one thermoplastic, having about 2% to about 15%, by volume, of reinforcing particles. The particles each comprise one or more layers. At least 50% of the reinforcing particles are less than about 20 layers thick, at least 99% of the reinforcing particles are less than about 30 layers thick, and the layers comprise platelets having a thickness of between about 0.7 nm and 1.2 nm. The thermoplastic and reinforcing particles are intimately mixed via the rotational action of at least one screw housed in an extrusion barrel. The melt, including 2%-15% by volume reinforcing particles, is conveyed under pressure along the surface of at least one screw to a metering section from whence the melt is forced to exit through a shaping die at the discharging end of the extrusion barrel. The discharged portions of the melt are then cooled and cut to desired lengths.